Apparatus and method for managing storage copy services systems

ABSTRACT

An apparatus for controlling a storage system having a data replication function, comprises: a storage array component being operable to send notification to a replication engine that a write of data to a primary storage location by a host is subject to data replication; the replication engine being operable to receive the notification and in response to instruct the storage array to copy the data to a secondary storage location; wherein the data is copied to the secondary storage location unmediated by the replication engine.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.14/874,654, filed on Oct. 5, 2015, which is a Continuation of U.S.patent application Ser. No. 11/945,844, filed on Nov. 27, 2007, thecontents of both hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a technology for improved management ofstorage systems, and more particularly to a technology for managingstorage systems having storage copy services.

BACKGROUND OF THE INVENTION

One of the important services provided by modern storage controllerproducts is a copy services solution. Copy services include, but are notlimited to, remote copy, point in time copy, and continuous dataprotection (CDP). These copy services are today typically implementedinside storage controllers as an integral part of the storage controllermicrocode. Recently, though, a new approach has emerged in the industrywhereby copy services are implemented in an appliance which will here becalled a “Replication Engine”, rather than inside a storage controller.

All the above copy services, or replication, functions rely on “writes”of the protected data being “split”. What this means is that the copyservices appliance receives a notification that a write is ongoingsynchronously with the write being performed to the storage controller.

In systems according to the prior art there are, broadly speaking, twodistinct implementations of this splitter technology. These arrangementsare shown in FIG. 1. Firstly, there is the technique of using a splitterimplemented in the fabric. This is shown on the left-hand side of FIG.1, where host 100 is connected to SAN fabric 104, and SAN-based splitter116 performs primary I/O to physical storage 106. SAN-based splitter 116also directs I/O to replication engine 110, in which the copy servicescomponent 112 resides and performs copy services. The second techniqueis to implement a splitter in the host software stack, such as thedevice driver. The host operating system's logical volume manager (LVM)can also be used for this function. This arrangement is shown on theright-hand side of FIG. 1, where splitter 114 is connected to host 102above the fabric level 104. In each of these implementations, the writedata is sent from the splitter to the replication engine.

The disadvantages of all these schemes are:

1. Multiple different implementations are required to cover all hosttypes and switch types.

2. Splitting in the host consumes host CPU MIPS and doubles writebandwidth requirement on the host to switch links.

3. Hosts and switches typically do not have access to non volatilememory. This means that it is hard for the hosts to reliably keep trackof the state of in-flight writes, forcing design compromises whicheither impact performance, robustness or the speed at which the solutioncan recover from loss of power.

U.S. patent application Ser. No. 11/840,179 discloses a technique thatpermits implementation of a generic splitting protocol inside theStorage controller rather than in the host or in the storage areanetwork (SAN). This protocol provides the infrastructure to build acommon Copy Function across several different Storage Controllers. Theinterface is intended to be a simple interface that makes it possible toconnect storage arrays (SA) to replication engines (RE) and to eachother and allow new replication functions to be more rapidly deployed.This implementation relies on the use of a protocol in which each writecommand received by the splitter is duplicated and sent in parallel toboth the primary storage and also to the replication engine. Thereplication engine has some storage, usually (but not required to be)provided by the same storage array that contains the splitter. The REuses this as a “repository” to implement its copy services. Thisrepository will typically contain the data that has been written to theprimary disks, together possibly with older copies of the data which wason the primary disks at some time in the past, together with metadata.In this protocol, the commands used transfer both control informationand the data that was received from the host. The expectedimplementation of a replication engine is that it will not include diskstorage but will instead use storage LUNs that are provided by thestorage array. Thus the data flow for a simple write which the RE justneeds to store one copy of in its repository is: Host→storagecontroller→Replication Engine→storage controller→disks (repositorystorage). Of course in reality, the RE will probably need to associatesome metadata with the data and may also need to copy data from oneplace in its repository to another place in the repository. It may alsoneed to copy data between the primary disks and the repository.

Such an arrangement is illustrated in FIG. 2, in which are additionallyillustrated a storage appliance 200 having a splitter 202 and a storagevirtualization controller 204 having a splitter 206.

The data flow for a split write according to all of theseimplementations of the prior art may be shown in simplified form asillustrated in FIG. 3, in a which a host 300 writes data at flow 1 tostorage array 302. Storage array 302 flows the data to the primaryphysical storage 306 at flow 2 and to replication engine 304 at flow 3.Replication engine 304 adds metadata and flows the data and metadata tothe storage array 302 at flow 4, and storage array 302 flows the data tothe secondary physical storage 308 at flow 5.

Flowing the data through the RE has the following disadvantages:

-   -   The data has to make at least an extra two passes across the        network and across the busses that connect the Storage array        memory to the network. This limits the bandwidth that can be        achieved by the solution for any given hardware platform.    -   If the storage controller implements a data format including        check-bytes for data integrity checking then either the RE has        to participate in the scheme or the data is unprotected in this        part of the data flow.

It would thus be desirable to have an improved technology for managingstorage systems having storage copy services.

SUMMARY OF THE INVENTION

The present invention accordingly provides, in a first aspect, anapparatus for controlling a storage system having a data replicationfunction, comprising: a primary storage location; a secondary storagelocation; a replication engine; and a storage array component coupled tothe primary storage location, the secondary storage location, and thereplication engine, the storage array component configured to receive,from a host, a write command to write data to the primary storagelocation, write the data to the primary storage location, and to send acommand descriptor block (CDB) describing the write command, withoutsending the data, to the replication engine in response to receiving thewrite command, the CDB indicating that the data is subject to datareplication in the secondary storage location, wherein: the replicationengine is configured to receive the CDB and, in response thereto,generate metadata for protecting the data based on the CDB and transmitthe generated metadata to the storage array component.

Embodiments of the present invention, in its broadest aspect, provideapparatus and methods for exchanging control sequences and metadata withthe replication engine to allow it to implement copy services withoutthe data itself flowing through the replication engine.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described,by way of example only, with reference to the accompanying drawingfigures, in which:

FIG. 1 shows in schematic form two possible arrangements of apparatusaccording to the prior art;

FIG. 2 shows in schematic form two possible arrangements of apparatusaccording to one proposed improvement to the prior art;

FIG. 3 shows in simplified schematic form the flows of data for a splitwrite according to the prior art schemes illustrated in FIGS. 1 and 2;

FIG. 4 shows in simplified schematic form the flows of data for a splitwrite according to a preferred embodiment of the present invention;

FIG. 5 shows in simplified schematic form the flows of data for a dataread from an historic view of a volume preserved using CDP according tothe prior art;

FIG. 6 shows in simplified schematic form the flows of data for a dataread from an historic view of a volume preserved using CDP according toa preferred embodiment of the present invention;

FIG. 7 shows in simplified schematic form the flows of data for a datawrite to an historic view of a volume preserved using CDP according to apreferred embodiment of the present invention; and

FIG. 8 shows in simplified schematic form the flows of data for aresynchronization of a primary volume with a secondary volume accordingto a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will now be described,with reference to the figures. As described above, FIGS. 1 to 3 depictarrangements of apparatus and data flows according to the prior art.These have been described above as forming the background to the presentinvention.

In very broad terms, the preferred embodiments of the present inventionprovide hardware and software according to a new and improved protocol.The protocol informs the replication engine of all new writes to volumesto which the replication engine has subscribed. An interface is providedwhich allows the replication engine to direct the newly written data toan area on the replication engine repository volumes. The interface alsoallows metadata generated by the RE to be added to the data as it iswritten to the repository and allows data movement between therepository and the primary volumes. The protocol allows for thenecessary serialization of these data movements and allows consistencygroups to be created between volumes. It also allows volumes to becreated to access a “copy services view” of the data in the repositorysuch as a historical view of data in a CDP engine.

As will be clear to one of ordinary skill in the art, an SA→RE→SA dataflow is necessary when the RE needs to obtain a copy of the data,perhaps to send it to a RE at a remote site (Global Mirror/MetroMirror).However, provided that the new and improved protocol according to apreferred embodiment of the present invention is used, for functionssuch as Continuous Data Protection (CDP) and snapshot or T0 copying,there is no need for the server's data to actually flow through thereplication engine. The preferred embodiment of the present inventionprovides a scheme to avoid this.

For the purposes of the description of preferred embodiments of thepresent invention, any “in band virtualization appliance” such as theIBM System Storage SAN Volume controller is treated as a storage array(SA). The term “storage array” is not therefore limited to the merephysical arrangement of storage media, but encompasses the entirety ofthe hardware and software provision for the control of and storage ofdata in the array.

Some advantages of using the preferred embodiments of the presentinvention rather than the splitter protocol described above are:

-   -   As the data does not have to flow into the RE and then back out,        the requirements for bandwidth are much reduced in the SA, the        SAN and the RE. For a given hardware system this will allow        significantly higher data rates to be achieved.    -   As the data does not have to flow through the RE, the scheme        works even for SAS that embed data integrity fields into the        data. The RE does not need to know that the SA has done this and        does not need to have support for this.    -   As the data does not flow between the SA and RE, the link        between the SA and the RE can be implemented using a lower        bandwidth interconnect technology. Potentially, the link could        be implemented using Ethernet rather than fibre channel. This        may save significant development expense and time to market for        a storage array that does not currently implement a Fibre        Channel initiator.

The basic premise is that a “splitter” will be written in the StorageArray (SA) that will essentially mirror the write data between theprimary volume and the RE. It is intended that the storage used by theRE will be on the same SA appliance as that is managing the primaryvolume.

The exemplary descriptions of the Figures given below explain how the SAcommunicates customer write data to the RE and how the RE provides CDPfor the customer data. The command sequences are intended to be vendorspecific SCSI commands, rather than the standard SCSI commands. A SCSI“Read” is thus not meant to indicate a real SCSI read, but a commandthat looks like a read with the direction of the data transfer goingfrom the target to the initiator (SA being the initiator).

One known example of an RE product wraps Host customer data with headerand footer information. This information is the RE metadata—the RE usesthis information to manage the CDP'd primary volume data accordingly. REdevices may also choose to stripe data the CDP data and Metadata acrossthe LUNs, and therefore some of the commands require lists of LUNs andLBAs.

Turning now to FIG. 4, there is shown in simplified schematic form theflows of data for a split write according to a preferred embodiment ofthe present invention. Host 300 flows the write data to the storagearray (SA) 302 at flow 1, and SA 302 flows the write data to the primaryphysical storage 306 at flow 2. Completion is returned by primaryphysical storage 306 at flow 2 a, and SA 302 returns completion to host300 at flow 2 b. The command descriptor block (CDB) for the write issent by SA 302 to replication engine (RE) 304 at flow 3. RE 304generates metadata and returns it to SA 302 at flow 4. The statusmessage is also sent by RE 304 to SA 302 at flow 5. SA 302 combines thewrite data and the metadata received from RE 304 and sends thecombination to secondary physical storage 308 at flow 6. It is thusclear to one of ordinary skill in the art that, advantageously, the flowof data to and from the RE 304 is avoided, with consequent benefits inprocessing and bandwidth reductions. To ensure that the write data ispreserved in case of power loss while it is “in-flight”, all in-flightwrite data must be preserved in non-volatile storage between flow 2 band flow 6 of FIG. 4.

The Status message sent across the network is used for cross splitterconsistency groups, where it is possible that an RE may have beendisconnected from another SA.

In the case of a Read issued to an historic view of a primary volume(that is, an instruction to read data, not from the current up-to-datelayer, but from layer representing the data at a specified time in thepast), a similar beneficial effect can be achieved by the preferredembodiment of the present invention.

Turning to FIG. 5, there are shown the command and data flows accordingto the prior art for this situation.

In FIG. 5, host 300 issues a READ command specifying an historic view atflow 1. SA 302 flows the READ command to RE 304 at flow 2. RE structuresthe command according to its data respecting the historic view and flowsthe READ to SA 302 at flow 3. SA 302 issues the READ to secondaryphysical storage 308 at flow 4, and secondary physical storage returnsthe data to SA 302 at flow 5. The data is passed by SA 302 to RE 304 atflow 6 to satisfy the RE's READ, and RE returns the data to SA 302 atflow 7. SA 302 then returns the data to host 300 at flow 8.

Turning to FIG. 6, there are shown the command and data flows accordingto the preferred embodiment of the present invention for this situation.

In FIG. 6, host 300 sends a READ command to SA 302, which in turn sendsthe READ command to RE 304 at flow 2. RE 304 sends a status message atflow 3 to SA 302, which issues the READ to secondary physical storage308 at flow 4. Secondary physical storage 308 returns the requested datato SA 302 at flow 5, and SA returns the requested data to host 300 atflow 6.

It is possible that the RE may have fragmented or striped the dataacross multiple LUNs or that the RE-assigned LUN has become fragmentedwithin the snapshot area and hence there is a requirement for the RE tobe able to specify multiple locations of the host data. Thus the READthat is flowed to the SA 302 from the RE 304 may be a multiple-locationREAD.

There are occasions when it becomes necessary to issue a write to asecondary volume—for example, when it is necessary for the system torevert to data in an historic view preserved using the CDPfunctionality. This is the case, for example, when later data has becomecorrupted and it is necessary to “turn the clock back” in order tocontinue processing on the basis of earlier, uncorrupted data.

Turning to FIG. 7, there are shown the command and data flows accordingto the preferred embodiment of the present invention for this situation.

In FIG. 7, at flow 1, host 300 sends data to be written to SA 302, whichin turn sends the write CDB to RE 304 at flow 2. RE 304 sends metadataat flow 3 and a status message at flow 4 to SA 302, which combines thewrite data and metadata and writes it to secondary physical storage 308at flow 5.

In the case when an RE volume (or LUN) becomes out of synchronizationwith the primary volume (e.g., due to disconnection, it is necessary toresynchronize the RE LUN with the primary volume. During any period ofdisconnection between an RE and an SA, the SA is obliged to continueprocessing write I/O requests for the primary volume. While it isdisconnected the primary volume will then become out of synchronizationwith the RE LUN that is storing the CDP data. When a reconnection isdetected by the SA, the SA needs to send its DRL (Dirty Region Log) tothe RE so the RE knows that writes were received by the SA during thedisconnection. The RE then initiates copy requests for each of theregions of the primary volume that are dirty.

The command and data flows for this situation are illustrated in FIG. 8,in which host 300 has been processing write I/Os to primary physicalstorage 306 during a period when SA 302 and RE 304 have beendisconnected—possibly owing to a network disruption or a local failureat the system housing the RE 304. When the connection is re-established,RE 304 requests resynchronization at flow 1. SA 302 responds by sendingits Dirty Region Log (DRL) to RE 304 at flow 2. RE 304 sends metadata toSA 302 at flow 3. SA 302 then sends a READ command to primary physicalstorage 306 at flow 4, and receives the returned data from primaryphysical storage 306 at flow 5. SA 302 applies the metadata to the dataand sends the resulting data and metadata combination for each regionthat was out of synchronization to secondary physical storage 308 atflow 6.

In similar fashion as for normal write as described above with referenceto FIG. 4, the RE needs to wrap header and footer information around thecustomer data. The SA initiates the read requests to the primary volume,merges the data with the header and footer information supplied by theRE and then initiates the write to the RE LUN storing the CDP data.

The preferred embodiment of the present invention includes a facilityfor providing configuration instructions to the RE. Some examples ofsuch configuration commands are:

-   -   Revert/Rollback primary volume to time point X    -   Create a history view of a volume, time point X    -   Snapshot Primary volume    -   CDP protect this primary volume    -   Create space efficient volume    -   Delete space efficient volume

For consistency group support where a single RE may be used for multipleSAs, so that SAs are alerted when consistency group synchronization hasbeen broken, a heartbeat mechanism is provided between the RE and theSAs.

In summary, then, some important aspects and advantages of the preferredembodiment of the present invention are as follows:

1. There is a facility for the SA and RE to discover each other on thefabric over which they will communicate or to be configured to know eachother's addresses.

2. There is a facility for the SA and the RE to be configured so thatthe SA knows which primary volumes are configured for copy services, howto split to the RE and how to address the RE.

3. The SA can be configured with some storage space that is for use bythe RE as a repository.

4. The SA can be configured to send a message to the RE when a new writecommand is addressed to a volume for which copy services are configured.This message contains the address being written (target, LUN/LBA) andthe length being written but not the data.5. When the RE receives notification that a new write has been receivedthere is a facility to allow the RE to copy this written data to zero ormore places in the repository area. The protocol also allows the RE towrite metadata into the repository area associated with the server data.6. There is a facility to prevent the data being overwritten before theRE has had a chance to copy it. In a preferred embodiment, the RE isallowed to copy the data whilst the write command is still active in theSA and to prevent two simultaneous write commands from writing to thesame disk blocks. In this case the RE has to acknowledge the new writebefore the SA allows the write to complete.7. There is a facility to allow the RE to copy data in either directionbetween the repository and the primary volumes and from one place toanother on the repository.8. In case the connection between the RE and the SA is broken it isimportant that the protocol implements a way to detect and recover. Onesuch way is to implement a heartbeat protocol so that the RE can knowthat the SA has become disconnected and can know that any writesreceived from other SAs may be “dependent” upon writes that it has notbeen informed about. In addition to the heartbeat, the SA must keep arecord of any writes that have been received and not sent to the RE. Itis acceptable for the SA to coalesce this information up so that only abitmap of regions is kept indicating which regions have been written toand must be synchronized.9. Optionally, the protocol can provide a facility to intercept reads sothat it is possible to present data to the server that comes from thecopy services repository. In this case the protocol provides a facilityto intercept reads and return data as directed by the RE.

It will be clear to one of ordinary skill in the art that all or part ofthe method of the preferred embodiments of the present invention maysuitably and usefully be embodied in a logic apparatus, or a pluralityof logic apparatus, comprising logic elements arranged to perform thesteps of the method and that such logic elements may comprise hardwarecomponents, firmware components or a combination thereof.

It will be equally clear to one of skill in the art that all or part ofa logic arrangement according to the preferred embodiments of thepresent invention may suitably be embodied in a logic apparatuscomprising logic elements to perform the steps of the method, and thatsuch logic elements may comprise components such as logic gates in, forexample a programmable logic array or application-specific integratedcircuit. Such a logic arrangement may further be embodied in enablingelements for temporarily or permanently establishing logic structures insuch an array or circuit using, for example, a virtual hardwaredescriptor language, which may be stored and transmitted using fixed ortransmittable carrier media.

It will be appreciated that the method and arrangement described abovemay also suitably be carried out fully or partially in software runningon one or more processors (not shown in the figures), and that thesoftware may be provided in the form of one or more computer programelements carried on any suitable data-carrier (also not shown in thefigures) such as a magnetic or optical disk or the like. Channels forthe transmission of data may likewise comprise storage media of alldescriptions as well as signal-carrying media, such as wired or wirelesssignal-carrying media.

A method is generally conceived to be a self-consistent sequence ofsteps leading to a desired result. These steps require physicalmanipulations of physical quantities. Usually, though not necessarily,these quantities take the form of electrical or magnetic signals capableof being stored, transferred, combined, compared, and otherwisemanipulated. It is convenient at times, principally for reasons ofcommon usage, to refer to these signals as bits, values, parameters,items, elements, objects, symbols, characters, terms, numbers, or thelike. It should be noted, however, that all of these terms and similarterms are to be associated with the appropriate physical quantities andare merely convenient labels applied to these quantities.

The present invention may further suitably be embodied as a computerprogram product for use with a computer system. Such an implementationmay comprise a series of computer-readable instructions either fixed ona tangible medium, such as a computer readable medium, for example,diskette, CD-ROM, ROM, or hard disk, or transmittable to a computersystem, via a modem or other interface device, over either a tangiblemedium, including but not limited to optical or analogue communicationslines, or intangibly using wireless techniques, including but notlimited to microwave, infrared or other transmission techniques. Theseries of computer readable instructions embodies all or part of thefunctionality previously described herein.

Those skilled in the art will appreciate that such computer readableinstructions can be written in a number of programming languages for usewith many computer architectures or operating systems. Further, suchinstructions may be stored using any memory technology, present orfuture, including but not limited to, semiconductor, magnetic, oroptical, or transmitted using any communications technology, present orfuture, including but not limited to optical, infrared, or microwave. Itis contemplated that such a computer program product may be distributedas a removable medium with accompanying printed or electronicdocumentation, for example, shrink-wrapped software, pre-loaded with acomputer system, for example, on a system ROM or fixed disk, ordistributed from a server or electronic bulletin board over a network,for example, the Internet or World Wide Web.

In one alternative, the preferred embodiment of the present inventionmay be realized in the form of a computer implemented method ofdeploying a service comprising steps of deploying computer program codeoperable to, when deployed into a computer infrastructure and executedthereon, cause said computer system to perform all the steps of themethod.

In a further alternative, the preferred embodiment of the presentinvention may be realized in the form of data carrier having functionaldata thereon, said functional data comprising functional computer datastructures to, when loaded into a computer system and operated uponthereby, enable said computer system to perform all the steps of themethod.

It will be clear to one skilled in the art that many improvements andmodifications can be made to the foregoing exemplary embodiment withoutdeparting from the spirit and scope of the present invention.

What is claimed is:
 1. An apparatus for controlling a storage systemhaving a data replication function, comprising: a primary storagelocation; a secondary storage location; a replication engine; and astorage array component coupled to the primary storage location, thesecondary storage location, and the replication engine, the storagearray component configured to receive, from a host, a write command towrite data to the primary storage location, write the data to theprimary storage location, and to send only a command descriptor block(CDB) describing the write command, without sending any portion of thedata, to the replication engine in response to receiving the writecommand, the CDB indicating that the data is subject to data replicationin the secondary storage location, wherein: the replication engine isconfigured to receive the CDB and, in response thereto, generatemetadata for protecting the data based on the CDB and transmit thegenerated metadata to the storage array component such that the metadatais generated after the data has been written to the primary storagelocation yet prior to replicating the data to the secondary storagelocation, and the storage array component is configured to add thegenerated metadata to the data as the data and the generated metadata isbeing written to the secondary storage location.
 2. The apparatus asclaimed in claim 1, wherein the replication engine is configured toutilize the generated metadata to manage protection of the data writtento the primary storage location and copied to the secondary storagelocation.
 3. The apparatus as claimed in claim 1, further configured toperform a read of the data, wherein the data is returned by the storagearray component to the host without being transmitted to the replicationengine.
 4. The apparatus as claimed in claim 1, further configured towrite data from the host to the secondary storage location and whereinthe data is written by the storage array component to the secondarystorage location unmediated by the replication engine.
 5. The apparatusas claimed in claim 1, further configured to resynchronize data betweenthe primary storage location and the secondary storage location andwherein the data is transferred by the storage array component from theprimary storage location to the secondary storage location unmediated bythe replication engine.
 6. A method for controlling a storage systemhaving a data replication function, comprising the steps of: receiving,by a storage array component, a write command to write data to a primarystorage location from a host; writing the data, by the storage arraycomponent, to the primary storage location; sending, by the storagearray component, only a command descriptor block (CDB) describing thewrite command, without sending any portion of the data, to a replicationengine in response to receiving the write command, the CDB indicatingthat the data is subject to data replication in a secondary storagelocation; receiving the CDB by the replication engine; generatingmetadata for protecting the data based on the CDB and in response toreceiving the CDB such that the metadata is generated after the data hasbeen written to the primary storage location yet prior to replicatingthe data to the secondary storage location; transmitting the generatedmetadata from the replication engine to the storage array component; andadding, by the storage array component, the generated metadata to thedata as the data and the generated metadata is being written to thesecondary storage location.
 7. The method as claimed in claim 6, furthercomprising managing the data written to the primary storage location andcopied to the secondary storage location, by the replication engine,utilizing the generated metadata.
 8. The method as claimed in claim 6,further comprising performing a read of the data, wherein the data isreturned by the storage array component to the host without beingtransmitted to the replication engine.
 9. The method as claimed in claim6, further comprising writing data from the host to a secondary storagelocation and wherein the data is written by the storage array componentto the secondary storage location unmediated by the replication engine.10. The method as claimed in claim 6, further comprising resynchronizingdata between the primary storage location and the secondary storagelocation and wherein the data is transferred by the storage arraycomponent from the primary storage location to the secondary storagelocation unmediated by the replication engine.
 11. A computer programproduct, stored on a non-transitory computer-readable medium, comprisingcomputer program code to, when loaded into a computer system andexecuted thereon, cause the computer system to control a storage systemhaving a data replication function, said computer program codeperforming the steps of: receiving, by a storage array component, awrite command to write data to a primary storage location from a host;writing the data, by the storage array component, to the primary storagelocation; sending, by the storage array component, only a commanddescriptor block (CDB) describing the write command, without sending anyportion of the data, to a replication engine in response to receivingthe write command, the CDB indicating that the data is subject to datareplication in a secondary storage location; receiving the CDB by thereplication engine; generating metadata for the data based on the CDBand in response to receiving the CDB such that the metadata is generatedafter the data has been written to the primary storage location yetprior to replicating the data to the secondary storage location;transmitting the generated metadata from the replication engine to thestorage array component; and adding, by the storage array component, thegenerated metadata to the data as the data and the generated metadata isbeing written to the secondary storage location.
 12. The computerprogram product as claimed in claim 11, further comprising managing thedata written to the primary storage location and copied to the secondarystorage location, by the replication engine, utilizing the generatedmetadata.
 13. The computer program product as claimed in claim 11,further comprising performing a read of the data, wherein the data isreturned by the storage array component to the host without beingtransmitted to the replication engine.
 14. The computer program productas claimed in claim 11, further comprising writing data from the host toa secondary storage location and wherein the data is written by thestorage array component to the secondary storage location unmediated bythe replication engine.
 15. The computer program product as claimed inclaim 11, further comprising resynchronizing data between the primarystorage location and the secondary storage location and wherein the datais transferred by the storage array component from the primary storagelocation to the secondary storage location unmediated by the replicationengine.